The present invention relates to a light beam focusing apparatus for use in a disk reproducing apparatus, and particularly to a focus error detection apparatus employing a trisection photodetector.
Disk players for reproducing information recorded on an information recording disk such as a video disk, a digital audio disk, or the like, employ focusing servo apparatuses for maintaining accurate focus of an information detection light beam on a recording surface of the disk. In such focusing servo apparatuses, the focus error of a beam convergent lens for focusing a light beam on the recording surface of the disk is detected, and the position of the beam convergent lens along an optical axis of the light beam is controlled in accordance with the detected focus error.
A number of methods have been used for detecting focus error, including what are referred to as an astigmatism method, a beam-diameter measurement method, a knife-edge method, and a critical-angle method. Of these methods, both the astigmatism method and the beam-diameter measurement method employ a belt-like photodetector having a light reception surface which is divided into three sections. These sections generally are positioned consecutively with respect to what may be referred to as a reference axis. The light beam striking such a photodetector will cause three photodetector outputs to be provided in accordance with the position of the light beam with respect to the sections of the photodetector.
FIG. 6 shows an example of an optical system which is part of a focus error detection apparatus, and exemplifies the astigmatism method. In the Figure, a light beam is emitted from a light source 1, such as a laser diode. The light beam is made incident onto a beam convergent lens 3 through a beam splitter 2, and thus is converged onto a recording surface of a disk 4. A light beam reflected from the recording surface of the disk 4 passes again through the beam convergent lens 3 and strikes a portion 2a of the beam splitter 2 so as to be reflected towards a cylindrical lens 5.
The lens 5 imparts astigmatism to the reflected beam, and causes the thus altered beam to strike a light reception surface of a photodetector, such as the photodetector 6 shown in FIG. 7 of the application. Such photodetector is disposed with respect to the cylindrical lens 5 such that the light reception surface of the photodetector has a position wherein, with respect to two focal lines L.sub.1 and L.sub.2, shown in FIG. 6, a cross section of luminous flux of the light beam output by the lens 5 is circular.
As shown in FIG. 7, a focus error FE is obtained in accordance with the equation P.sub.A -(P.sub.B +P.sub.C). This expression is obtained as follows. The light reception outputs P.sub.B and P.sub.C are added in an adder 7, and the output of the adder 7 is provided to a negative input of an differential amplifier 8. The output P.sub.A of the central section A of the photodetector 6 is provided to a positive input of the differential amplifier 8.
In accordance with the astigmatism method mentioned above, if the beam convergent lens 3 is displaced from a position of focus, the shape of the light beam will become elliptical, as shown in FIGS. 8(A) and 8(C). A properly focused beam would be circular, and would appear as shown in of FIG. 8(B). In any event, an unfocused beam would cause the sum of the outputs P.sub.B +P.sub.C either to increase (FIG. 8(A)) or decrease (FIG. 8(C)).
On the other hand, in accordance with the beam-diameter measuring method mentioned above, an optical system slightly different from that used for the astigmatism method is employed, and the results may be described with reference to FIG. 9(A-C). A properly-focused beam would appear as in FIG. 9(B). An improperly-focused beam may appear as shown in FIG. 9(A)-9(C), so that the diameter of an unfocused beam would be larger or smaller than desired. Either of the situations in FIGS. 8(A), 8(C), 9(A) or 9(C) will provide a focus error FE which is nonzero, whereas the situations in FIGS. 8(B) and 9(B) will provide a focus error FE equal to zero (that is , P.sub.A -(P.sub.B +P.sub.C)=0).
In the apparatuses employed in each of the above-mentioned focus error detection methods, each of the widths b and c of the outer light reception surface sections B and C of the photodetector 6, as measured along an axis perpendicular to the above-mentioned reference axis (that is, in the direction perpendicular to the dividing lines in the light reception surface) is selected to be at least as large the width a of the central light reception surface section A. Thus, in the case where the center of the light beam and the center of the light reception surface do not coincide, and in fact deviate along the above-mentioned second axis, a non-zero focus error signal FE will be generated in accordance with the size of that deviation.
For example, assume a light beam strikes the photodetector as shown in FIG. 10, which shows some deviation of the center of the light beam from the center of the photodetector. As the center of the light beam is moved with respect to the center of the photodetector, outputs of the various sections A-C may be depicted as shown in FIG. 11. In that figure, the abscissa represents the degree of separation of the center of the light beam from the center of the light reception surface, given a light beam radius equal to 1, and the ordinate represents the output level of the light reception surfaces of FIGS. 7 and 10. The output level may be set to 1 when the light reception surface receives all of the light beam output, as shown in FIG. 7. In this case, each of the widths b and c of the photodetector 6 are set equal to the width a of the central light reception surface section (a=b=c=0.808). Selection of those width values makes the amount of light received in the central portion A equal to the sum of the respective amounts of light received by the sections B and C when the light beam is properly focused, as shown in FIGS. 8(B) and 9(B). As can be seen in FIG. 11, for the various photodetector outputs P.sub.A, P.sub.B, and P.sub.C for the structure of the photodetector shown in FIGS. 7 and 10, there is a very limited range wherein the difference P.sub.A -(P.sub.B +P.sub.C) is close to 0.
Looking at a different situation, wherein the photodetector has a shape shown in FIG. 12 of the application, in which the widths b and c are set to a value equal to 2.0, a relation of focus error signal to deviations of light beam center from the center of the photodetector may be depicted as shown in FIG. 13. Again, there is a very small range bounding the zero point on the abscissa wherein the difference P.sub.A -(P.sub.B +P.sub.C) is substantially equal to 0.
Thus, in conventional apparatuses, it is difficult to obtain a stable focus error signal FE, because even relatively slight deviations of the center of the light beam from the center of the photodetector as measured along the above-mentioned second axis can cause great variations in the level of the focus error signal.